I use powdered Quietex along with Su-Per Mag Calm on Casper, but that's only part of the regimen.
I've taken him completely off sugar laden feeds, and gone to beet pulp and an alfalfa based dry pellet.
He also stays out 24/7/365, unless the weather's just so horrendous it would be detrimental to his health to be out.
All of that, along with regular exercise and ear poofs, have made a world of difference in his attitude.
None of the calmers are going to be a miracle cure. You have to take everything
into account with a reactive, spooky horse and adjust their life accordingly.